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<h2 align=center>free</h2>
<h4 align=center>OS/161 Reference Manual</h4>

<h3>Name</h3>
<p>
free - release/deallocate memory
</p>

<h3>Library</h3>
<p>
Standard C Library (libc, -lc)
</p>

<h3>Synopsis</h3>
<p>
<tt>#include &lt;stdlib.h&gt;</tt><br>
<br>
<tt>void</tt><br>
<tt>free(void *</tt><em>ptr</em><tt>);</tt>
</p>

<h3>Description</h3>
<p>
<tt>free</tt> releases a block of memory previously allocated with
<A HREF=malloc.html>malloc</A>, <A HREF=calloc.html>calloc</A>,
or <A HREF=realloc.html>realloc</A>.
</p>

<p>
Once free has been called, <em>ptr</em> is no longer valid and
attempts to dereference it result in undefined behavior.
(Pedantically, in fact, even using the <i>value</i> of
<em>ptr</em> may produce undefined behavior.) Passing <em>ptr</em> to
free a second or subsequent time (unless of course the same pointer
value is again returned from malloc) is also undefined and
particularly likely to provoke adverse behavior in most
implementations.
</p>

<p>
<tt>free(NULL)</tt> has no effect.
</p>

<p>
In practice it is desirable for implementations of <tt>free</tt> to
detect, to the extent practically possible, pointers that were not
previously allocated by one of the above functions or that are passed
to <tt>free</tt> multiple times. However, this can be difficult and
there is no useful standard mechanism for error reporting.
</p>

<p>
<tt>free</tt> does not necessarily unmap free memory or return it to
the operating system, but may do so if it chooses.
</p>

<h3>Return Values</h3>
<p>
<tt>free</tt> returns no value.
<p>

<h3>See Also</h3>
<p>
<A HREF=calloc.html>calloc</A>,
<A HREF=malloc.html>malloc</A>,
<A HREF=realloc.html>realloc</A>
</p>

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